The discussions are taking place around a point raised by Jacob Rees-Mogg relating to article 62 of the Vienna convention.
Photograph: Dan Kitwood/Getty Images

Senior Tory Eurosceptics believe they and the Democratic Unionist party could be persuaded to back Theresa May’s Brexit deal if Geoffrey Cox, the attorney general, gave clearer legal advice about how the UK could withdraw from an international treaty.

It is understood the DUP is back in talks with senior government figures about what it would take for them to back May’s deal at a third Commons vote. A party source said: “Channels are open.”

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The majority of Eurosceptic MPs from the European Research Group (ERG) voted against May’s revised deal, defeating it for a second time, because Cox advised there was only a “reduced risk” that the UK could be trapped indefinitely in the Northern Ireland backstop and therefore a customs union with the EU.

However, discussions are taking place around a point that Jacob Rees-Mogg, the ERG chair, raised in the House of Commons before Tuesday’s vote, relating to “how article 62 of the Vienna convention could be used”.

Stephen Barclay, the Brexit secretary, replied that the UK would have the ability to terminate the withdrawal agreement “if the facts clearly warranted that there had been an unforeseen and fundamental change of circumstances affecting the essential basis of the treaty on which the United Kingdom’s consent had been given”.

He added: “It would, in the government’s view, be clear in those exceptional circumstances that international law provides the United Kingdom with a right to terminate the withdrawal agreement. In the unlikely event that that were to happen, the United Kingdom would no doubt offer to continue to observe the unexhausted obligations in connection, for example, with citizens’ rights.”

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An ERG source said this had been written by Cox but had not made it into the final legal advice. “If we’d had it earlier in the day it could have changed the vote,” the source said.

He said not everyone in the ERG had the potential to be convinced, but many still wanted to back an improved deal rather than leave on no-deal terms. If there was some more momentum in favour of a deal, a few more Labour MPs could potentially be convinced.

No 10 is still holding out hope of getting the deal passed through the Commons on a third or even fourth attempt.

Earlier on Wednesday, Steve Baker, the ERG’s chief organiser, told BBC Radio 4’s Today programme that the group had been “really yearning” to vote for a deal but Cox’s bombshell legal advice on the Northern Ireland backstop meant that was impossible.

Rees-Mogg also did not rule out changing his mind on the deal in future.

Speaking to the Guardian’s Today in Focus podcast, due out on Friday, Rees-Mogg said: “I think leaving without a deal is better than the prime minister’s deal as it is currently announced, though I would have preferred a deal had that proved possible and if this deal could be made more palatable I would still prefer a deal to no deal. It is not a cunning plan to get us to no deal by default.”

Asked whether he would vote for the deal if the DUP were happy with it, he said: “Yes. I am a unionist but it is difficult for me to be more unionist than the DUP and if they are happy with the way of getting out of the backstop and that is good enough for Northern Ireland then I expect it would be good enough for some of us.”

Asked whether he would vote for May’s deal on 28 March if it was a choice between that and losing Brexit, Rees-Mogg said: “In the words of the deceased prime minister Lord Oxford and Asquith, wait and see.”